This invention relates to novel compounds designed to bind to the penicillin receptor, to methods for preparing the novel compounds, and to the use of the compounds as antibacterial agents.
Many antibiotics act by interfering with the biosynthesis of bacterial cell walls.1 The completion of bacterial cell wall synthesis is mediated by enzymes termed penicillin-binding proteins (PBPs)2 which crosslink different peptidoglycan chains. In particular, PBPs link the penultimate D-Ala residue of a peptidoglycan terminating in a N-acyl-D-Ala-D-Ala moiety to the terminal amino group of a lysine residue of another peptidoglycan chain. Glycopeptide transpeptidase is an example of a PBP present in many bacteria.
All known PBPs contain a conserved Ser-X-X-Lys sequence at the active site. The xcex2-lactam family of antibiotics, whose members include penicillins and cephalosporins, inhibit PBPs by forming a covalent bond with the active site serine residue. In the case of penicillin, the labile xcex2lactam ring reacts with the hydroxyl group of the active site serine to form an acyl-enzyme intermediate as shown in Scheme 1 below. 
The enzyme is therefore unable to carry out the final step in the biosynthesis of bacterial cell wall.3 As a result, the wall is weakened, becomes permeable to water, and the bacterial cell swells, bursts, and dies.
The simplest kinetic description of the reaction between a bacterial enzyme (Enz) and a xcex2-lactam antibiotic is given in Scheme 2 below: 
In addition to the PBP""s, many bacteria also produce a second type of penicillin-recognizing enzyme, known as a xcex2-lactamase. PBPs and xcex2-lactamase enzymes exhibit the same kinetics as set forth in Scheme 2 above, but with different rate constants.4 This difference in rate constants has important consequences. In the case of the PBP""s, k2 greater than  greater than k3 (i.e the formation of the acyl-enzyme is much faster than its hydrolysis). The result is that the enzyme is inhibited, and antibacterial activity may be observed. In the case of a xcex2-lactamase, k2≈k3 (i.e. the formation and hydrolysis of the acyl-enzyme proceed at comparable rates). These kinetics lead to regeneration of the enzyme, and inactivation of the antibiotic as a result of the net hydrolysis of the xcex2lactam bond in the deacylation step. The latter sequence of reactions comprises the principal mechanism of bacterial resistance to xcex2-lactam antibiotics. Useful antibacterial activity is considered to require k2/k1xe2x89xa71000M xe2x88x921 secxe2x88x921 and k3xe2x89xa61xc3x9710xe2x88x924 secxe2x88x921.
Resistance to antibiotics is a problem of much current concern.5 Alternatives to existing antibiotics are invaluable when bacteria develop immunity to existing drugs or when patients are allergic to existing drugs (approximately 5% of the population is allergic to penicillin). Because of the relatively low cost and relative safety of the xcex2-lactam family of antibiotics, and because many details of their mechanism of action and the mechanism of bacterial resistance are understood, one approach to the problem of resistance is to design new classes of compounds that will complex to and react with a penicillin-recognizing enzyme, and be stable to the hydrolysis step. In order to be effective, the antibacterial agent should have the ability to react irreversibly with the active site serine residue of the enzyme.
The crystal structures of xcex2-lactamases from B. licheniformis, S. aureus and E. coli (RTEM) suggest a chemical basis for resistance to xcex2-lactam antibiotics (FIG. 1). Apart from the conserved Ser-X-X-Lys active site sequence, xcex2-lactamases have a conserved Glu166 which participates in the hydrolysis of the acyl-enzyme. It appears that the hydroxyl group of the active site serine and the carboxyl group of Glu166 together with a water molecule are involved in the hydrolysis step.6, 7 The inventors have found that, in water solvent, one xe2x80x9cnon-spectatorxe2x80x9d water molecule plays an active role in the carboxylic acid catalysis of ester hydrolysis.6,7 The water molecule and the carboxyl group act in concert and this interaction is the source of bacterial resistance to xcex2-lactam antibiotics. Drug design must therefore include a process for the removal or inactivation of this water molecule.
Numerous xcex2-lactam antibiotics have been developed in the past which are structural analogues of pencillin and which complex to and react with penicillin-recognizing enzymes. Like penicillin, such antibiotics are presumed to be conformationally constrained analogues of the N-acyl-D-Ala-D-Ala peptidoglycan moiety, the Oxe2x95x90Cxe2x80x94N xcex2-lactam bond being a bioisostere of the D-Ala-D-Ala peptide bond.8 Effective antibacterial activity also requires a properly positioned carboxyl group or equivalent and a hydrogen-bonding hydroxyl or acylamino group. One of the inventors has previously designed a computer-implemented molecular modelling technique for identifying compounds which are likely to bind to the PBP active site and are therefore likely to exhibit antibacterial activity. This modelling technique is described in U.S. Pat. No. 5,552,543 issued Sep. 3, 1996, the disclosure of which is hereby incorporated by reference.
FIG. 2 summarizes comparative chemical reactivity results obtained using an improvement to the computer strategy described in the 543 Patent. The activation energies (kcal/mol) for the reactions of various ring systems with a hydroxyl group were calculated and compared to that of the bicyclic ring system of penicillin. The inventors have determined that these relative reactivities parallel exactly the relative antibacterial activities of all known classes of xcex2-lactam compounds (as well as non-xcex2-lactam compounds, such as Lactivicin9 and the pyrazolidinone family of synthetic antibiotics10, which are known to complex to and react with penicillin-recognizing enzymes). The ring system identified in FIG. 2 as A formed the basis for synthesis of novel compounds exhibiting antibacterial activity described in the 543 Patent. The present application is directed to novel oxazinone compounds based on the 1,2oxazine ring system identified in FIG. 2 as B.
Some oxazinones having possible biological activity are known in the prior art. Khomutov et al synthesized tetrahydro-1,2-oxazin-3one (Chemical Abstracts 13754a, 1962) and 4-benzamidotetrahydro-1,2-oxazin-3one (Chemical Abstracts 58, 13944b, 1963). The latter compound is also known as N-benzoyl-cyclocanaine. According to Khomutov, cyclocanaline is known to inhibit glutamate-aspartate tramsaninase and exhibits activity against tuberculosis bacilli. The structure of cyclocanaline is shown in formula (A) below. 
Frankel et al reported the synthesis of DL-cyclocanaline (4-amino-tetrahydro-1,2-oxazin-3-one) hydrochloride from canaline dihydrochloride in 1969 (J. Chem. Soc. (C) 1746-1749, 1969) and recognized that DL-cyclocanaline is a higher homologue of the antibiotic cycloserine.
Barlos et al reported the synthesis of S-4-(N-tritylamino)-tetrahydro-1,2-oxazin-3-one (also known as N-trityl-cyclocanaline) in 1988 (Liebigs Ann. Chem. 1127-1133, 1988).
The inventors are not aware of any reports of cyclocanaline derivatives or other oxazines which have been previously recognized to be structural analogues of penicillin. The need has therefore arisen for a new class of oxazinones synthesized to exhibit antibacterial activity which satisfy the following structural requirements:
1 The compound will complex to the active site of a penicillin recognizing enzyme.
2. The compound includes a functional group that, when positioned properly at the active site of the enzyme, is able to react with the hydroxyl group of the active site serine by a mechanism and with a rate constant comparable to the mechanism and rate constant exhibited by penicillin or cephalosporin.
3. The resulting acyl-enzyme is stable and resistant to hydrolysis, thereby preventing regeneration of the bacterial enzyme at a significant rate.
In one aspect, the invention provides a compound selected from the group consisting of compounds having antibacterial activity represented by the general formula (I) and pharmaceutically acceptable salts thereof: 
where R1 is the side chain of a D- or L-alpha amino acid, R2 is OH, NH2, or NHCOR3, R3 is a substituent known to confer antibacterial activity when present in the side chain of a penicillin or cephalosporin, R4 is H or loweralkyl, R5 is one of OH, NH2 or NHCOR3 when R6 is H, and R6 is one of OH, NH2 or NHCOR3 when R5 is H. Preferably R5 and R6 taken together comprise the oxygen of a carbonyl group which absorbs a water molecule found at the active site of a xcex2-lactamase enzyme.
In one embodiment R2 is OH, R1, R4, R5 and R6 are hydrogen and the compound has the S-configuration at C5. In another embodiment, the compound has the R-configuration at C5. The compound may have either the S-configuration or the R-configuration at C1xe2x80x2.
A process for the production of the compound represented by formula (I) is also provided comprising condensation of a carboxyl-protected N-hydroxy alpha-amino acid with a 3-hydroxy-protected-4-bromobutanoic acid, cyclization of the resalting doubly protected N-hydroxy-N-acylated alpha amino acid, and removal of the protecting groups. In one embodiment the carboxyl-protecting group is t-butyl, the hydroxyl-protecting group is 2-tetrahydropyranyl, the condensing agent is dicyclohexylcarbodiimide, the cyclization is performed with an organic amine, and the removal of the protecting groups is performed with trifluoroacetic acid.
The invention also pertains to a pharmaceutical composition comprising an effective amount of a compound according to formula (I) together with a pharmaceutically-acceptable carrier and a method of treatment of bacterial infection in a mammal, comprising the step of administering to a mammal in need of such treatment an effective amount of a compound according to formula (I).